1. Field of the Invention
This invention relates to multi-channel access (MCA) cordless telephones of the type generally described in copending application Ser. No. 403,477, filed Sept. 6, 1989 and assigned to the same assignee as the present invention; and, more particularly, to an MCA-type cordless telephone system in which several remote stations, such as handset units, may be queried simultaneously for the purpose of establishing a communication channel with one of them, within a brief period of time.
2. Description of the Prior Art
Cordless telephone systems of the multi-channel access type which rely upon relatively low electrical power requirements have been proposed heretofore. In one such proposal, eighty-seven separate communication channels and two separate control channels are provided, each channel being a "cordless" communication path having a particular carrier frequency onto which information and data are modulated. To expand the functions of an MCA cordless telephone system, a so-called "multi-cordless" arrangement has been introduced in which a master station is comprised of plural base units, each including a set of transmitting and receiving circuits adapted to communicate with one of several remote stations, such as handset units.
An example of the aforementioned multi-cordless telephone arrangement is illustrated in FIG. 1 in which one master station, such as base unit 2, is adapted to communicate with plural remote stations, such as the eight handset units 1A, 1B, . . . 1H that are illustrated. Base unit 2 is connected to a conventional telephone line 3 and is adapted to communicate with the handset units by way of radio transmission between the antenna provided at the base unit and each of the antennas provided at the handset units.
In the arrangement shown in FIG. 1, when an incoming telephone call is received from telephone line 3 to base unit 2, the base unit signals all of handset units 1A-1H at the same time, whereupon one of the handset units responds and, thus, answers the incoming telephone call. Typically, the following techniques are used to signal the handsets and complete a telephone call:
In one technique, base unit 2 signals each of handsets 1A-1H individually and in sequence. At the completion of the entire sequential signalling operation, that is, after the last handset has been signalled, the base unit awaits the response from a handset which wishes to answer the telephone call.
Another technique relies upon simultaneous signalling of all of the handsets, with the expectation that only one handset, that is, the desired handset unit, will respond.
As yet another technique, all of handset units lA-1H are signalled at the same time, but the signalling identifies a particular handset unit as a "representative handset unit" which is the only unit enabled to respond. Thus, although other "non-representative" units receive the signal from base unit 2, they are inhibited from responding thereto and, thus, are not able to answer the incoming telephone call.
All of the aforementioned techniques suffer from various disadvantages and drawbacks. The first technique, which signals individual handset units in sequence, requires an unusually long period of time to complete the sequential signalling and to answer the incoming telephone call. This is because the handset units are not maintained in a steady receive-enable mode of operation so as to detect base unit signalling. Rather, in an effort to minimize the power requirements of the handset units (due to the fact that these units typically are energized by a battery), a handset unit typically is disposed in a quiescent or "sleep" mode from which it is "awakened" to its receive mode only periodically. For example, and as shown in FIG. 2A, in a 2-second period, the handset unit may be disposed in its receive mode for only 0.2 seconds, while remaining in its quiescent, power-down mode for the remaining 1.8 seconds. Thus, a handset unit is enabled only briefly to receive the signalling transmitted from base unit 2. Furthermore, the periodicity illustrated in FIG. 2A, which illustrates the changeover between the quiescent and receive modes of a handset unit, is not synchronized in all of handset units 1A-1H. Although the repetition rates of these changeover operations may be about the same, the instantaneous times at which the handset units are changed over from their quiescent mode to their receive mode are random.
Accordingly, to ensure that a signal from base unit 2 will be detected by each of handset units 1A-1H, the base unit transmits this signal periodically over a predetermined interval to handset unit 1A, then to handset unit 1B, and so on. As illustrated in FIGS. 2A and 2B, the signal transmitted from the base unit may exhibit a duration of only 120 milliseconds, but this signal is repealed continuously for a period of approximately 2.5 seconds to make certain that the signal is present when handset unit 1A is disposed briefly in its receive mode. It is expected that the handset unit receives the signal from base unit 2 at a time t2, whereafter this handset unit does not return to its quiescent mode but, rather, remains in its receive mode, as represented by the broken line shown in FIG. 2A. Following the end of the 2.5 second signalling interval, which occurs at time t3, the signalled handset unit responds at, for example, time t4, as illustrated in FIG. 2C. This "answer back" signal is delayed by an amount & following the end of the signalling interval. The "answer back" signal is detected at base unit 2 to establish a communication channel between the base unit and the responding handset unit (assumed herein to be handset unit 1A).
In a typical MCA cordless telephone system, the signalling from base unit 2 to a handset unit and the "answer back" from that handset unit are carried out over a control channel which, typically, is separate and distinct from the communication channel. Since the use of the control channel for other purposes generally is not permitted, a handset unit is inhibited from responding to the signalling from the base unit until all of the handset units have been signalled. Once this signalling is completed, the base unit awaits the receipt of an "answer back" from one of the handset units, whereafter a call indicator at that handset unit, such as a ringing device, is energized. It is appreciated that if 2.5 seconds are needed to signal one handset unit successfully, twenty seconds (2.5 seconds .times.8 handset units) are needed to call all of the handset units. This unusual time delay is unacceptable, especially since an incoming telephone caller may discontinue his telephone call long before it is completed on the assumption that the called party (i.e. base unit 2 in combination with handset units 1A-1H) is not available to "answer".
In the second signalling technique mentioned above, wherein all of the handset units are signalled simultaneously, the time needed to establish a communication channel between the base unit and one handset unit is substantially reduced from that just described. However, it is not unusual for two or more handset units to transmit "answer back" signals to the base unit at time t4. As a result, radio interference between these simultaneously transmitted "answer back" signals prevents the base unit from determining which handset unit wishes to establish the communication channel. Hence, communication is prevented.
In the third of the aforementioned techniques for establishing communication between the base unit and a handset unit, wherein only the representative handset unit is permitted to response to the signalling, the drawbacks generally attending the other two techniques are avoided. However, if the representative handset unit is unable to receive the signal transmitted from base unit 2, or if the representative handset unit is unable to communicate with the base unit, for example, if its battery power is low, if it has malfunctioned or if another cordless telephone in close proximity therewith monopolizes the control channel, or if it is located beyond the effective communication range with the base unit, the signalling process is stopped. That is, base unit 2 is unable to establish communication with the representative handset unit or with any other handset unit. Consequently, an incoming telephone call cannot be "answered" if there is no reply from the representative handset unit, regardless of the reason for that lack of reply.